Alloyed nodular iron



May 5, 1959 c. D. DICKINSON ALLOYED NODULAR IRON 2 Sheets-Sheet 1 Filed Aug. 22, 1957 HEAT NO. 2

ALLOYED CAST 1w HEAT NO I2 HEAT NO I ALLOYED UNALLOYED NODULAR IRON CAST IRON m wwR NWR TILA ALL MW UO ALLOYED NODULAR- IRON 00 w 0 54 2 63 mm som awn 32 m R A L U 3w O N N mm m I... m U

D 8 6 4 30a 0006 wmwmkw IO HOURS STRESS RUPTURE VALUES OF UNALLOYED MODULAR IRON AND ALLOYED NODULAR IRON United States PatentG ALLOYED NODULAR IRON Clayton D. Dickinson, Warren, Ohio, assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis.

Application August 22, 1957, Serial No. 679,679 6 Claims. (Cl. 75-124) This invention relates to a nodular cast iron alloy and more particularly to an improved nodular iron alloy containing aluminum and other ferrite forming alloying additions consisting of one or more of the elements of the group comprising molybdenum, tungsten, zirconium, titanium and silicon and possessing superior elevated temperature properties in the as-cast condition.

To a considerable extent, the present invention is based on the discovery that an extraordinarily high order of elevated temperature properties, such as the stress rupture strength, oxidation resistance, and tensile strength are obtainable by introducing specified quantities of aluminum and other ferrite forming elements in a molten nodular iron bath. As an example, when the present nodular iron alloy of this invention is compared with a similar unalloyed cast iron, an increase of more than 530 percent in the stress rupture strength at the 1,000 hour level at 1200 F. results. When a gray cast iron having base composition comparable to the aforementioned nodular iron, is alloyed in accordance with this invention, the stress rupture strength at the 1,000 hour level at 1200 F. is increased approximately sixty-eight percent as will be hereinafter more fully described. The stress rupture strength and other physical properties of a nodular cast iron alloyed in accordance with the present invention are significantly increased.

An as-cast alloy of the present invention contains over two percent aluminum but not more than nine percent of aluminum. The additional ferrite forming constituents fall within the range of about .1 percent to ten percent. The carbon content of the alloy may be varied considerably without adversely afiecting the high order of properties. However, the carbon must be present in nodular or compacted form.

In the past many attempts have been made to improve gray cast iron by adding various alloying elements includ ing aluminum and molybdenum. In the patent, US. 2,134,905 to James W. Bampfylde, it is disclosed that a cast iron alloy containing from 0.5 percent to nine percent aluminum possesses improved mechanical strength at temperatures as high as 500 C. and that at higher temperatures the mechanical strength diminishes (see column 2 beginning at line 11 of the patent to Bampfylde). In the prior art, aluminum has not been favorably regarded as a possible alloying element in nodular iron because it has an adverse effect on the casting properties (see column 8 beginning at line 40 of the patent to Millis et al., US. 2,485,760). The use of molybdenum as an alloying element to improve the mechanical properties of alloy cast iron alone or in combination with nickel or the use of cerium as a nodularizing agent is likewise well known in the prior art.

In striking contrast to the cast irons of the prior art containing the aforementioned alloying elements, those of the present invention are charatcerized and distinguishable by the fact that they possess a much higher order of elevated temperature properties than would ordinarily be expected from any prior teaching or practice. It has long ice been recognized that the mechanical properties of cast iron may be generally improved by the use of the various alloying elements. So far as it is known, there has been no information published or any discovery made that a nodular iron alloyed in accordance with the teaching of the present invention will increase the mechanical properties at elevated temperatures to the significant extent as will hereinafter be more fully described. Likewise, it has never been previously shown that such markedly improved properties could be obtained in the as-cast condition of the nodular iron without sacrificing the machinability of the casting.

It is an object of the present invention to provide a nodular cast iron containing aluminum, in combination with another ferrite forming addition consisting of one or more of the group comprising molybdenum, silicon, titanium, tungsten and zirconium and characterized by greatly improved elevated temperature properties.

It is a further object of the present invention to provide a nodular cast iron having improved mechanical and physical properties at elevated temperatures and being machinable in an as-cast condition.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following description, taken in conjunction with the drawings in which:

Fig. 1 is a photograph of a specimen of the alloyed nodular iron of the present invention (Heat No.12) and three other specimens of comparable cast irons (Heat Nos. 1, 2 and 3) after being exposed in an oxidizing atmosphere at 1800 F. for a period of hours;

Fig. 2 illustrates the stress rupture versus time curves for alloyed nodular iron prepared according to the present invention (Heat Nos. 4 and 5) and an unalloyed nodular iron (Heat No. 3);

Fig. 3 illustrates the tensile strength versus temperature curves for an alloyed nodular iron prepared in accordance with the present invention (Heat No. 12) and three other comparable cast irons (Heat Nos. 1, 2 and 3); and

Fig. 4 illustrates the oxidation resistance curves (weight gain or loss in grams per square inch versus hours of exposure at 1800 F.) for an alloyed nodular iron prepared in accordance with the present invention (Heat No. 12) and three other comparable cast irons (Heat Nos. 1, 2 and 3).

In the as-cast condition the alloy of the present invention possesses the characteristic microstmcture of nodular iron. It will accordingly be seen that this product is a new ferrous material distinguishable from ordinary nodular cast iron in that the material exhibits a high order of superior elevated temperature properties and similar to ordinary nodular cast iron in that it retains the inherent characteristics of the basic nodular cast iron. Accordingly, the alloying elements introduced to improve the mechanical and physical properties must not interfere with the formation of the spherular granules of graphite in the microstructure of the cast iron.

The preparation of an alloy according to present invention will be described in connection with a known process for producing nodular iron which was used in the preparation of the various alloys exemplifying the present invention. The process used is the one disclosed in the patent to H. K. Ihrig, US. 2,750,284, and is referred to herein as the salt process. It should be readily apparent to one skilled in the art that the alloy which is the subject of the present invention can be produced by any of the other methods used to make nodular iron. It is not intended to limit the present invention to any specific process of making nodular iron.

The composition required for the cast iron to be alloyed in accordance with this invention depends also upon what substances are present in the cast iron inaddition to the iron and carbon. Cast iron, as the term is used herein, applies to a ferrous alloy which has sufficient carbon when the metal is solidified and cooled from the liquid state to form a microstructure characterized by free carbon inclusions in the cold metal. Since the presence of silicon and certain other substances affects the transformation of cementite into ferrite and free carbon, the range of carbon content of the basic ferrous metal, which is to be alloyed in order to obtain the improved results of this invention, will vary to some extent. Broadly state, a range of carbon sufficient to produce a nodular iron will be a satisfactory base metal to which the alloying elements of the present invention may be added. With reference to the salt process of making the nodular iron used to produce the examples illustrating the superior properties of the present alloys, it was found that a range extending from 0.08 percent carbon to 6.7 percent carbon will give satisfactory results.

The process of producing the present alloy, in its simplest form, involves the addition of appropriate amounts of the alloying elements, the molten aluminum, and the mixture of the nodularizing agents, magnesium chloride and calcium silicide, into the molten cast iron. The actual sequence of the melting operation may be varied as desired. As an example, the ferrite forming addition may be added directly to the base melt. To obtain a desired molybdenum content in the examples used to exemplify the invention, the ferromolybdenum was added directly to the melt after the melt was heated to a temperature of 2800 F. The metal was then deoxidized with calcium silicon and the slag was removed. The aluminum and the required amount of nodularizing agents were then placed in a preheated receiving ladle and the molten base metal poured from the furnace over them. Depending upon the type of melting furnace employed, the mixture of the aluminum and the nodularizing agents may be added directly into the molten iron in the ladle.

Another satisfactory variation in the procedure is to melt the aluminum separately and pour it into the ladle with the molten iron. This practice may be desirable where it is necessary to minimize the chilling effect of the solid aluminum addition in the ladle. If the sulfur content of the iron is too high, it is first desulfurized by any well known method such as by adding sodium carbonate or calcium oxide to the molten iron.

A preferred procedure is one which is adaptable to a cupola operation. The base metal is heated in the cupola to a temperature of approximately 2800 F. At the same time, a measured quantity of the aluminum to be alloyed is heated to approximately 1400 F. in a separate crucible. The molten aluminum is then added to a pouring ladle where the ferromolybdenum or other alloying elements, the ferrocerium and the nodularizing agents are added. The molten base metal or iron is then poured into the ladle. The heat of the solution of the aluminum in the iron is suflicient to maintain the temperature of the final alloy at approximately 2800 F.

Since the reaction of the molten base metal and the molten additions take place with moderate violence, it is found that stirring is not necessary. As the metal is poured, considerable care must be exercised to pour the metal continuously to prevent the formation of a skin of aluminum oxide on the exposed surface of the melt.

It is known that the presence of lead, bismuth, tin, arsenic and antimony will interfere with the formation of the spheroidal form of carbon. To insure against the subversive effect of these elements, a small amount of cerium may be added. It should also be noted that the cerium is an active desulfurizing agent. To preserve the effectiveness of the cerium addition, it is important that the base metal or iron be low in sulfur or the cerium will combine the sulfur to form ceriumsulphide and float on the surface of the melt.

Although the use of molybdenum is described as one of the alloying elements in the exemplifications of the improved product obtainable by the practice of this invention, it is not intended that the composition be restricted to this element. It should be readily apaprent to one skilled in the art that other substitutions of elements to the base composition which produces or helps in producing the ferritic matrix of the nodular iron in the as-cast condition can be used to successfully practice the present invention. Other strong ferrite and weak carbide forming elements, such as silicon, titanium and others may be substituted for the molybdenum or used in combination with it.

It will be evident that certain properties of the present alloy can be enhanced and modified by subjecting the alloy to the conventional heat treatments. A homogenization heat treatment at a temperature between 1900 and 1950 F. for approximately four hours followed by slow furnace cooling to 1200 F. overnight and then cooling to room temperature produces an alloy of improved ductility. Such a heat treatment produces an alloy having a more homogeneous structure and also permits the transformation of the carbides which might be present.

The nodular cast iron alloy obtainable by virtue of this invention provides a metallic engineering material having an exceptionally high stress rupture strength at 1200 F. In comparison to the increase obtained by alloying a cast iron having a comparable composition, the alloy of the present invention shows an increase in the stress rupture strength 780 percent greater at the 1,000 hour stress level. The alloyed nodular iron also shows a significant increase of approximately 290 percent in stress rupture strength at the hour and 1,000 hours levels over a comparable unalloyed nodular iron.

In order that the improvement obtained by the present composition be more fully appreciated by one skilled in the art, oxidation tests were conducted on comparable specimens of a plain gray cast iron, a plain nodular cast iron, a cast iron containing the alloying elements prescribed herein and a nodular cast iron also containing the alloying elements prescribed herein. The superior oxidation resistance of the alloyed nodular iron is graphically presented in the photograph of Fig. 1 showing the four specimens after being exposed for approximately hours to a temperature of 1800 F.

To further exemplify the superior order of properties obtainable in the alloyed nodular iron of the present invention, a comparison was made with a comparable alloyed cast iron, an unalloyed nodular iron, and an unalloyed cast iron of comparable base compositions. The tensile strength of the alloyed nodular cast iron is from 104 to 116 percent greater than the tensile strength of the plain nodular iron at temperatures ranging from 1000 F. to 1400F. At 1400 F. the tensile strength of both the plain nodular-and plain cast iron specimens were found to be the same. Creep tests conducted on comparable samples ofthe alloyed nodular iron, alloyed cast iron, unalloyed nodular iron and unalloyed cast iron also give further supporting evidence to the exceptional behavior of the alloyed nodular iron at elevated temperatures.

Notwithshtanding the distinctly novel properties of the alloy of the present invention, it should be understood that the addition of the alloying elements described herein have been a subject of common discussion in the technical literature. However, there is nothing in the past literature that suggests or predicts the high order of mechanical and physical properties possessed by a nodular iron alloy containing the elements in accordance within the proportions set forth herein.

An an illustrative example of a heat prepared according to the present invention, a charge of pig iron weighing approximately 108.5 pounds and containing 3.5 percent carbon, 2.5 percent silicon,.0.5 percent (maximum) manganese, 0.05 percent (maximum) phosphorus, 0.05 percent (maximum) sulfur was melted and heated to 2800 F. At this point 1.15 pounds of ferromolybdenum (62.5 percent molybdenum) was added to the melt. To deoxidize the melt 0.39 pound of calcium silicon was added, and the oxides floating on the surface were skimmed. The following materials were then placed in a preheated receiving ladle: 4.6 pounds of aluminum containing less than one percent impurities, 6.3 grams flint metal (twenty-five percent iron and seventy-five percent misch metal), 0.74 pound of anhydrous magnesium chloride and 1.4 pounds of calcium silicon. The molten metal from the furnace was then poured over the mixture of these materials. The ladle was stirred and skimmed. The castings were then poured immediately. It was found that optimum pouring temperatures ranged from 2500 F. to 2550" F.

The melting sequence described in the above example is illustrative of a typical sequence followed in the preparation of the numerous heats tested. This heat produced a nodular iron containing about three percent aluminum and one percent molybdenum. It should be noted that the addition of calcium silicide is not necessary if the iron does not need deoxidizing. If the base iron contains more than 0.05 percent sulfur, it will have to be first desulfurized by adding sodium carbonate or calcium oxide to the melt.

In order that those skilled in the art may have a better understanding of the present invention, stress rupture tests were conducted on numerous heats prepared. The chemical composition of the various heats tested is presented in Table I below:

TABLE I Chemical analyses of materials tested for comparative evaluation Heat No. C Si Mn Mo Al 3. s 1. 57 0.69 2. 75 1.67 0. 70 1.9 21 a 70 2. 36 0. a0 a. 11 3. 54 0. 2e 1. 96 s. 27 2. 0s a. 50 0. 1s 1. 72 5. 97 2. 84 4.12 0.17 1. 95 4. 99 3. 1s 3. 30 0. 15 1. 97 5. 01 3. 05 a. 64 1. 01 5. 04 3. 04 3.10 1. 07 5. 42 2.95 a. 24 1. 20 a. 90 a. 57 2. 44 0. so 1. 00 5. 2a a. 43 2.06 0. 3o 2. 20 6.35

The results of the tests conducted to determine the stress rupture strength is tabulated in Table II below for the 100 hour and 1,000 hour levels:

It is noted that Heat No. 4 containing approximately 6.27 percent aluminum and 1.96 percent molybdenum possesses a stress rupture life of 100 hours for an applied stress of 20,000 pounds per square inch and a stress rupture life of 1,000 hours for an applied stress of 14,000 pounds per square inch. It is further noted that this represents an approximate increase of 292 percent at the 100 hour level and an increase of 290 percent at the 1,000 hour level over an unalloyed nodular iron (Heat No. 3). The test results indicate a stress rupture life of hours for an applied stress of 5,100 pounds per square inch for the unalloyed nodular iron (Heat No. 3) and 3,600 pounds per square inch at the 1,000 hour level. Unalloyed cast iron (Heat No. 1) has a stress rupture strength of 3,270 pounds per square inch at the 100 hour level and 2,200 pounds per square inch at the 1,000 hour level. When the cast iron is alloyed with 5.21 percent aluminum and 11.9 percent molybdenum, a stress rupture strength of 5,370 pounds per square inch at the 100 hour level and a stress rupture strength of 3,700 pounds per square inch at the 1,000 hour level are obtained or an increase of sixty-eight percent over the comparable unalloyed cast iron heat.

To further illustrate the increased stress rupture values obtainable in a nodular cast iron alloy according to the present invention, the numerical values of the stress rupture were plotted against time in hours for the nodular iron alloy containing the alloying additions (Heat Nos. 4 and 5 the plain nodular iron without the alloying additions (Heat No. 3). This curve is shown in Fig. 2.

The striking contrast between the nodular iron alloy of this invention and other comparable alloys is further illustrated by the short time elevated temperature tensile tests conducted on samples of Heat Nos. 1, 2, 3 and 12. The results of these tests are tabulated in Table III:

TABLE HI Short time elevated temp rature tensile test results of comparative materials 1,000 F. 1,200 F. 1,400 F. 1,600 F.

Heat

T.S. E. T.S. E. T.S. T.S. E. (p.s.i) (per- (p.s.i.) (per- (p.s.i.) (per (p.s.l.) (percent) cent) cent cent) 1 T.S.'Iensile strength in pounds per square inch. E.-Elongation in 2 inches, percent.

For comparative purposes it is to be noted that at 1000 F., 1200 F. and 1400 F., the tensile strength of the alloy (Heat No. 12) of this invention is from 104 to 116 percent greater than the tensile strength of a plain nodular iron (Heat No. 3). In the same range of temperatures, the alloy of the present invention is from 104 to 160 percent greater than the tensile strength of a comparable plain cast iron (Heat No. 1). The results of the tensile strength tests are graphically shown in Fig. 3 where the tensile strength versus temperature curves are plotted for Heat Nos. 1, 2, 3 and 12.

By comparing the results of the oxidation tests conducted on samples of Heat Nos. 1, 2, 3 and 12, as shown in the curves of Fig. 4, one skilled in the art will readily appreciate that the present invention provides an engineering material having extraordinary high resistance to oxidation at 1800 F. The Weight gain or loss per square inch of surface area of the alloy nodular iron of this invention is comparably insignificant after hours at 1800 F. The physical appearance of the four oxidation samples after 135 hours at 1800 F. is shown in the photograph of Fig. 1. It is apparent from the photograph that the nodular iron alloy of the present invention has the superior physical appearance and has only a negligible amount of scaling.

In order that those skilled in the art may have a better understanding of the various ferrite forming elements that can be used as alloying elements, illustrative examples of alloys produced in accordance with the invention are given in Table IV:

TABLE IV Chemical analysis of heats tested Heat 0, S, Al Si, Mo, Ti,

N 0. Percent Percent Percent Percent Percent Percent 1 Contains 0.005 boron addition.

As indicated by the chemical analysis of the experimental heats made to produce the alloys, the percentage i TABLE V High temperature strength at 1200 F.

Rupture Reduc- Elon a- Heat No. Stress, Time, tion tion,

p.s.i. Hrs. Area, Percent Percent TABLE VI Tensile strength at elevated temperatures Ultimate Heat No. Temperature Tensile Elongation,

of- Strength, Percent p.s.i.

400 64,000 13 1,000 33, 500 1, 200 31,050 1, 200 42, 400 2. 5 14 1, 400 14, 250 23. 5 r 1,200 24,000 1. 0 15 l 1, 400 16,300 7.0 1,200 24, 700 0. 5 1, 400 26, 100 9. 5 1, 200 33, 250 10.0 17 1, 400 11,000 18.0 1,200 57, 250 5. 0 18 1, 400 17, 900 s. 0 1,200 56,000 6. 5 19 1, 400 18, 100 10. 5

It will be evident from the aforementioned illustrative examples that a number of nodular iron alloys may be made in accordance with this invention which exhibit the significantly superior properties at elevated temperatures.

It is to be noted that the minor constituents and impurities for example, phosphorus, cerium, or manganese are not listed in Tables I and IV and may be present in such amounts as occur in the commercial practice of making nodular iron or in amounts not adversely affecting the properties of the alloys.

The term nodular iron, as used in the description and the appended claims, defines a ferrous metal which contains graphite yielding carbon and in the as-cast condition has a microstructure characterized to a substantial degree by compacted graphite inclusions substantially in spheroidal in shape and includes within the range of its composition minor constituents and impurities which do not adversely affect the nodularization of the cast iron or its properties, and also those constituents which are required to bring about nodularization.

Although the present invention has been described in connection with several preferred embodiments, it is understood that modification and variations may be made without departing from the spirit and scope of the invention. Such variations and modifications apparent to those skilled in the art are considered to be within the purview and scope of the invention and the appended claims.

What is claimed is:

1. As a new article of manufacture, an alloyed nodular cast iron characterized by being machinable in an as-cast condition and having improved mechanical and physical properties at elevated temperatures and containing from about 2.0 to 9.0 percent by weight of aluminum and from about 0.1 percent to 10 percent by weight of the one or more of the group of elements consisting of molybdenum, tungsten, zirconium, titanium and silicon with the balance comprising nodular cast iron.

2. An alloyed nodular cast iron characterized by being machinable in an as-cast condition and having improved mechanical and physical properties at elevated temperatures and containing about 2.0 to 9.0 percent aluminum by weight and about 0.1 to 5.0 percent molybdenum by weight with the balance comprising nodular cast iron.

3. An alloyed nodular iron characterized by being machinable in an as-cast condition and having improved mechanical and physical properties at elevated temperatures and containing from about 2.0 to 9.0 percent aluminum by weight and from 0.01 to 1.0 percent titanium by weight with the balance comprising nodular cast iron.

4. An alloyed nodular cast iron characterized by being machinable in an as-cast condition and having improved mechanical and physical properties at elevated temperatures and containing from about 2.0 percent to 9.0 percent aluminum by Weight and about 1.0 percent to about 5.0 percent silicon by weight with the balance comprising nodular cast iron.

5. An alloyed nodular cast iron characterized by being machinable in an as-cast condition and having improved mechanical and physical properties at elevated temperatures and containing approximately 3.0 to 6.5 percent aluminum by Weight and approximately 0.1 to 3.0 percent molybdenum by weight with the balance comprising nodular cast iron.

6. An alloyed nodular cast iron characterized by being machinable in an as-cast condition and having an ultimate tensile strength at a temperature of 1200 F. of not less than 24,000 and not more than 56,000 pounds per square inch containing approximately 3.0 to 6.5 percent aluminum by weight, approximately 0.1 to 3.0 percent molybdenum by weight, approximately 0.01 to 1.0 percent titanium by weight with the balance comprising nodular cast iron.

(References on following page) 10 References Cited in the file of this patent OTHER REFERENCES UNITED STATES PATENTS Morral: Journal of the Iron and Steel Institute, V01. 2 4 5 7 0 ini et 1 Oct 25 1949 130, 1 34, NO. 2 pages 419-428. PllbliShd by the Iron ,4 5,7 Minis et 1 25, 1949 5 and Steellnstiwte, London, England- 2 2 20 C 15 1953 Case 6!; 31.2 Aluminum in Iron and Steel, 1953, tome pages 410-422. Published by John Wiley and Sons, Inc.,

New York, NY. 

1. AS A NEW ARTICLE OF MANAFACTURE, AN ALOYED NODULAR CAST IRON CHARACTERIZED BY BEING MACHINABLE IN AN AS-CAST CONDITION AND HAVING IMPROVED MECHANICAL AND PHYSICAL PROPERTIES AT ELEVATED TEMPERATURES AND CONTAINING FROM ABOUT 2.0 TO 9.0 PERCENT BY WEIGHT OF ALUMINUM AND FROM ABOUT 0.1 PERCENT TO 10 PERCENT BY WEIGHT OF THE ONE OR MORE OF THE GROUP OF ELEMENTS CONSISTING OF 